In earth formation well logging, it is often necessary to use nuclear logging techniques to measure earth formation characteristics. In one form of well logging, the borehole and earth formation are irradiated with neutrons from a chemical or accelerator source, and the populations of neutrons and/or gamma rays, that are generated in the formation, are detected at one or more locations some distance or distances from the source. The detected neutrons and/or gamma ray are counted and the counts are correlatable to several earth formation characteristics.
One such correlatable earth formation characteristic is formation density. These formation density measurements are recorded on density logs and have several benefits. The primary use for density measurements is to identify porosity. However, other uses for density measurements include identification of minerals in evaporate deposits, detection of natural gases, determination of hydrocarbon density, and calculation of overburden pressure and rock mechanical properties.
In formation density logging, a radioactive gamma ray source is displaced in a borehole and emits low-energy gamma rays of approximately 0.66 Mev into the formation. Gamma rays may be thought of as high-velocity particles that collide with the electrons in the formation. At each collision, a gamma ray loses some, but not all, of its energy to the electron, and then continues movement with diminished energy. This type of interaction is known as "Compton scattering". The scattered gamma rays reaching the detector, at a fixed distance from the source, are counted as an indication of electron density or formation density. The number of Compton-scattering (gamma ray) collisions is related directly to the number of electrons in the formation. A fewer number of collisions imply fewer electrons, which usually means a higher electron density. Consequently, the response of the density tool is determined essentially by the electron density (number of electrons per cubic centimeter) of the formation. Electron density is related to the true bulk density, which in turn depends on the density of the rock matrix material, the formation porosity, and the density of the fluids filling the formation pores. In earth formation well logging, formation density logging tools along with several additional logging tools are usually run in combination, forming a "tool string". This tool string permits several different measurements to be made in the same borehole simultaneously with a corresponding minimization of rig-time, cost and risk to the well and logging tools. Normally, the formation density tool is the "bottom" tool in the string and the logging direction of the tool string is upward.
The density tool is usually a nuclear-type logging tool that emits gamma rays into the earth formation. However, some of the other tools in the tool string may also be nuclear-type logging tools containing their own gamma-ray or neutron logging source(s) that also emit gamma rays into the formation. Therefore, unless care is taken, it is possible that one of these other nuclear logging tools, especially located adjacent the formation density tool, may irradiate the formation with gamma rays in such a way that the formation density measurements are affected when the density tool later traverses (passes) the same depths as the "last" (bottom) tool.
For typical logging tool strings, the sources and detectors of a given tool are located at a sufficiently far away distance from the other tools that there are no directly-received (detected)events(gamma ray) arising from other sources in the tool string. However, if one of the logging sources happens to be a high output, high energy neutron source, such as an electronic deuterium-tritium 14 MeV "accelerator" source, then it is possible that one of the other logging tools in the tool string will receive additional gamma-ray events indirectly from the high output source.
The additional gamma rays will be due to neutron-induced activation of elements in the formation and borehole environments. Activation is the process by which a normally stable (non-radioactive formation element) nucleus becomes radioactive, when bombarded with another nuclear particle (usually a neutron). Once activated this nucleus emits gamma rays which can be detected. This activation can have radioactive decay times (half-lives) on the order of several seconds to several minutes and thus can still be present in appreciable amounts when other tools in the logging string pass the same location in a borehole a few seconds to a few minutes later. Therefore, any significant neutron-induced activation activity would be seen as additional events by any gamma-ray-detecting logging tools and in particular the formation density logging tool which normally follows an accelerator-based tool in the tool string. Therefore, unless the additional gamma ray count-rate due to activation is somehow compensated for, it would distort the formation density measurement, causing it to read a higher-than-actual count-rate, and thus a lower-than-actual formation density.
In neutron activation, different elements such as silicon and oxygen can generate gamma-rays, when bombarded with neutrons, that have higher energy levels than their activation thresholds. Similarly, other elements, such as aluminum, may also be activated by absorbing a neutron once the neutron has slowed down (reduced energies to thermal level). For example, the emitted gamma-rays from the activated silicon and oxygen which start out at energies of 1.78 MeV and 6.13 MeV, respectively, are scattered to lower energies in the formation and borehole regions. In the case of silicon and oxygen, both elements have significant activation cross-sections as well as relatively long-lived activated states (2.2 minute and 7.1 second half-lives respectively). After scattering in the formation and borehole, the activation oxygen and silicon gamma-rays appear as low energy gamma rays and can be detected by the density tool and thereby effecting the gamma ray flux in the energy band used to determine the formation density measurement. For this reason, it is necessary to correct the count of gamma rays detected in order to provide accurate density measurements.
Historically, multiple simultaneous logging measurements did not present the activation problems occurring today. The previous neutron tools which have been used to irradiate the earth formations have contained "chemical" steady-state sources or fission type sources. Chemical sources did not create the present day problems primarily because of two factors: 1) Only a small percentage of the "chemical" neutrons have energies sufficient to activate any element commonly encountered while logging, and 2) the thermal flux (.about.1.times.10.sup.7 to .about.1.times.10.sup.8) is not high enough to activate appreciably any elements commonly encountered while logging. Therefore, virtually no distortions in the density tool count-rates, and hence density, have been historically observed when a neutron tool proceeds a density tool in the logging string.
However, many present generation tools contain non-chemical (accelerator) neutron sources having not only higher outputs but also having much higher neutron energies. In particular, these tools may contain accelerators of the type which generate monoenergetic 14 Mev neutrons. All of these neutrons, when generated, are above the activation energy thresholds for commonly occurring formation and borehole elements such as silicon, oxygen, aluminum, barium and other elements. Therefore, these activated elements can produce low energy gamma rays that are detected by the density tool. In addition, the higher total neutron output further excites elements activated by thermal neutron absorption (thermal activation). The half-lives of these elements can also be on the order of seconds to minutes. Therefore, as stated earlier, if not somehow compensated for, the density tool counting rate will be increased by these neutron-induced activities, resulting in erroneously low density readings.
There have been attempts to compensate for factors that affect formation bulk density. In one solution, U.S. Pat. No. 4,297,575 (Smith, Jr. et al), a logging instrument contains a gamma ray source and two gamma ray detectors. Measurements of the count-rate at each of the gamma ray detectors are made while passing a logging instrument through a well borehole. By appropriately combining the count-rates of the gamma rays of each detector by a predetermined relationship, measurements may be made of the earth formation compensated bulk density in the vicinity of the gamma ray source and detectors. Simultaneous graphical plots or well logs of the formation bulk density and other parameters of interest are recorded as a function of borehole depth.
A second solution corrects density logs that are affected by naturally occurring gamma rays and is related to the Smith et al, is U.S. Pat. No. 4,529,877 (Arnold). This patent provides a corrected density log having a correction for the adverse effects of gamma radiation from thorium, uranium and potassium ore bodies. The formation is irradiated with gamma-rays having an energy level of 0.663 Mev. A short spaced detector and a long spaced detector are used to detect gamma rays from the formation. A gamma ray spectrum is observed at one of the detectors and is broken down into four energy windows across the spectrum and count-rate signals are determined and corrected to separate naturally occurring gamma rays. This information may then be combined with count-rate information from the other detector, thereby yielding a compensated density log corrected from naturally occurring gamma rays.
Although these methods address density logging, several limitations are encountered when using these methods. Neither patent addresses the current problem of the effect on density measurements of detected gamma rays generated by non-density tool logging sources. Smith does not actually describe a method for correcting density measurements affected by gamma rays from non-density sources. Smith also focuses on determining the thickness of the casing around a borehole. Arnold addresses the correction of density measurements but only focuses on the effects from the minerals thorium, uranium and potassium ore. Correction density measurements affected by others minerals such as silicon are not addressed by Arnold. In addition, Arnold's method requires the use of two detectors to measure and correct density measurements. Also, in Arnold, a typical environment would be one of approximately several hundred gamma ray API units of radioactivity. However, techniques in present logging environments typically have several thousand API units of radioactivity.
Therefore, there remains a need for a method to correct density measurements that are affected by gamma rays from sources other than the density tool.